1. Field Of the Invention
The present invention relates to a method of manufacturing a dielectric-isolated semiconductor device having a high isolation breakdown voltage.
2. Description of the Prior Art
It is well known in the art that a thin layer of silicon is preferably formed on a dielectric substrate in order to attain good electrical insulation between electric components provided on the substrate, as well as to cause no irregularity on the substrate surface. A method of forming a silicon layer on a substrate having an insulation layer has been studied since around 1964 to attain the aforementioned object, and a method called dielectric isolation has been developed as summarized in J. Electrochem. Soc. Vol. 124, No. 1, 1977, pp. 5c-12c.
FIG. 1A to FIG. 1F are cross sectional views showing a conventional method of manufacturing a semiconductor device of a high breakdown voltage by dielectric isolation.
As shown in FIG. 1A, a silicon oxide film is first provided on the surface of a silicon substrate 20, and portions to define grooves in the silicon substrate 20 are removed by photoengraving process and silicon oxide film etching process to form a silicon oxide film pattern 23. The silicon oxide film pattern 23 is utilized as a mask to etch exposed surface parts of the silicon substrate 20 by an etching solution mainly composed of KOH, to define grooves 27 enclosed by (111) surfaces of a slow etching speed, as shown in FIG. 1B.
Then, the silicon oxide film pattern 23 is removed and thereafter n.sup.+ type diffusion is performed over the entire surface, to form an n.sup.+ type diffusion layer 28, as shown in FIG. 1C. Thereafter a thick silicon oxide film 21 is formed as shown in FIG. 1D. Then a polysilicon layer 22 of about 500 .mu.m in thickness is formed on the silicon oxide film 21, as shown in FIG. 1E. Finally the silicon substrate 20 is mechanically polished from its underside, so that monocrystal silicon island can be obtained on an insulation film formed by the silicon oxide film 21, as shown in FIG. 1F.
A large number of processes for improving this method have been reported. The feature of such processes also resides in growing on a silicon substrate a thick polysilicon layer for serving as a support substrate and polishing or etching of the silicon substrate to appropriate thickness.
In the conventional method of manufacturing a semiconductor device having a high breakdown voltage as hereinbefore described, extremely thick polysilicon growth of 500 .mu.m must be performed in order to form the polysilicon layer 22. Thus, productivity in the growth step is inferior while the wafer is extremely warped. Further, since the monocrystal silicon islands are formed on the underside of the silicon oxide film 21 by mechanical polishing, they are essentially inferior in thickness accuracy, while damage is left on the surface of the silicon substrate 20.